It is known to provide transport services for data over a transmission link using various telecommunication protocols. One such set of protocols, known as UMTS, is the European proposal for a third generation (3G) cellular network. It is inter-operable with the existing GSM network, and is notable for providing high speed packet-switched data transmission. UMTS transmission protocols are defined in a series of standards developed by the Third Generation Protocol Partnership (3GPP). These standards define a number of layers for the transmissions, broadly in line with the well known ISO seven layer definition. There is a physical layer, L1, a data link layer, L2, and a network layer L3. Layer 2 of this UMTS protocol stack includes procedures for handling data from or to a MAC (Medium Access Control) and higher layers. The data in the form of Transport blocks or sets of Transport blocks is encoded/decoded to offer transport services over a radio transmission link. A channel coding scheme provides a combination of error detection, error correcting, rate matching, interleaving and transport channels mapping onto/splitting from physical channels.
At a receiver, it is necessary to identify a channel format and decode the overhead and payload in the data to enable these functions of the coding scheme to operate. Suitable coding schemes are described in detail in the 3GPP standard 25.212 to which the reader is referred. One of the schemes for the receiver, described in this document, is blind transport format detection (BTFD). This is a scheme for detecting the format of the transport channels and in particular detecting a finish of a block of data for one channel. It makes use of the fact that the block is terminated by an error detection code, in this case a CRC (Cyclic Redundancy Check) code. The scheme uses this to test whether a given sequence of bits in the data stream could be a CRC code, for a block of data bits preceding the CRC bits. For different channel formats, the length of the block may differ. It may be coded in various ways including convolution coding.
Explicit blind transport format detection involves performing the recursive add-compare-select (ACS) process of a trellis decode over the maximum Transport Format (TF) length, storing trace-back information as one goes. This is followed by a series of speculative trace-backs and subsequent CRC checks starting from each position where a potential transport format could have terminated. This is summarised in FIGS. 1, 2 and 3. When a CRC pass is found the resulting decoded sequence has a high probability of being the correct one of the correct length and hence also the correct transport format. This implies the need for a convolutional coded data sequence that has a CRC appended to it prior to encoding. The series of tracebacks and subsequent CRC checks are carried out in order of the shortest tracebacks first.